1. Field of the Invention
This invention relates generally to plasma ion sources and more particularly to methods for shielding and cooling such plasma sources.
3. Description of the Prior Art
Focused ion beam (FIB) tools are used for nanometer scale precision material removal. The benefits of FIB tools include nano-scale beam placement accuracy, a combined imaging and patterning system for accurate part registration and pattern placement, and low structural damage of the area surrounding the removed volume. However, conventional FIB systems typically have a maximum removal rate of ˜5 μm3/s that limits their usefulness for removing volumes with dimensions exceeding 10 μm. Conventional FIB systems are further limited by the low angular intensity of the ion source, so at large beam currents the beam size dramatically increases from spherical aberration. For high beam currents and hence high removal rates of material, a high angular intensity is required, along with high brightness and low energy spread.
One solution is to use plasma ion sources. Inductively coupled plasma ion sources typically wrap an RF antenna about a plasma chamber. Energy is transferred by inductively coupling power from the antenna into the plasma.
Other applications for this type of plasma source include its use as the primary ion source for Secondary Ion Mass Spectrometry (SIMS), Ion Scattering Spectroscopy (ISS), focused and projection ion beam lithography, proton therapy, and high energy particle accelerators.
In all cases, a high power density is deposited into the plasma from the antenna in order to create a high density plasma, and the plasma is biased to a potential of several thousand volts with respect to ground.
Due to the high power density and high voltage bias applied to the plasma, two issues arise with devices using inductively coupled plasma ion sources: (a) how to keep the device cool, and (b) how to electrically insulate the plasma chamber. The heat issue arises because of the RF energy applied to the antenna and inductively coupled to the plasma chamber. The isolation issue arises because the plasma chamber is biased to a high voltage whereas the surrounding RF antenna and the electrostatic Faraday shield used to terminate the time-varying electric field and screen it from the plasma are typically biased to ground.
Accordingly, the need arises for new designs and methods that allow effective operation of plasma ion sources without overheating and with adequate insulation between elements biased to different voltages.